(_Cycadites megalophyllus._ _Bd._) _Wealden. Isle of Portland._ _a._ Central mass of cellular tissue, _b._ Circle of ligneous plates, _c._ Zone of cellular tissue, _d._ False-bark.] Mantellia. _Lign. 50, 51._--The fossil Cycadeæ of the Isle of Portland were first described botanically by Dr. Buckland, (Geol. Trans, vol. ii. 2d Series,) under the name of _Cycadeoidea_; of which Memoir the account in _Bd._ p. 404, is an abstract. M. Ad. Brongniart, considering these plants as a peculiar type, referred them to a new genus, which he did me the honour to name _Mantellia_ (_Prod. Veg. Foss._). These stems or trunks are from one to two feet in height; the circumference of the largest not exceeding three feet. The stem is subcylindrical, and the external surface covered with the rhomboidal scars formed by the attachment of the leaf-stalks, and which are widest in their transverse diameter. There are two species, which are readily distinguished by the form of the stems, and the difference in the size of the cicatrices left by the petioles. The most common kind is short, and spheroidal, and the leaf-scars are relatively large; its shape has caused it to be named "_Crows' nest_," by the quarrymen, who believe these plants to be nests that were built by crows in the trees of the petrified forest with which they are imbedded. The specific name (_nidiformis_) adopted by M. Brongniart, expresses this popular notion. _Lign. 50._ represents a fine example from the Portland Dirt-bed, which exhibits a structure altogether similar to that which characterizes the stems of recent cycadeous plants; namely, (_a_) a central mass of cellular tissue surrounded by circles of laminated ligneous rays or plates (_b_); then a zone of cellular tissue (_c_), and an external cylinder of false-bark (_d_). The mode of increase by buds, from germs in the axillæ of the petioles, as in the living plants, is also distinctly seen. [Illustration: Lign. 51. Mantellia cylindrica. (_Brongn._); 1/8 _nat._ (_Cycadites microphyllus. Bd._) _Wealden. Petrified Forest of the Isle of Portland._] The other species is subcylindrical, and the leaf-scars are much smaller and more regular than in _M. nidiformis_, indicating a more delicate foliage, as expressed by Buckland's specific name: that of M. Brongniart refers to the cylindrical form of the stem. This plant was higher and more slender than its associate. Numerous buds are seen in the axillæ of the petioles in the specimen figured. These fossils present, both externally and internally, a close relation to the bulbiform stems of the recent Cycadeæ, named _Encephalartos_, of South Africa.[132] [132] The fossil Cycads of the Isle of Portland are admirably described and illustrated in Dr. Buckland's Bridgewater Essay, (p. 497, and pl. lx. lxi.), and their internal structure is fully explained.